Optical sensors are commonly used in the field of printers, hand-held, desk-top or otherwise. Functions of sensors within this field usually include label detection, gap detection, skewness indicator, label length or width determination, etc. Advantages of these devices include their small size and durability. However, as with any device, there are also disadvantages, such as the certain level of sensitivity, precision or tolerance associated with each sensor which can vary greatly, ambient lighting can greatly influence the reception of the light emitted by the sensor, and the characteristics of the print media can vary greatly. The higher the sensitivity, or tighter the tolerance, the higher the cost of the sensor, another disadvantage.
In order to maintain reasonable costs on printers and attain considerable accuracy, prior art devices have tried different methods which use lower-cost sensors to achieve consistent repeatable results. However, sensors get dirty, decay over time, manufacturing techniques vary, and in many other ways the characteristics of each sensor are different or can change over time. Thus, prior art designs which did not precisely account for these variations or changing ambient conditions could not provide consistently reliable results. Other prior art designs offer manual adjustability or self-calibration but with heightened design and manufacturing complexity and greatly increased costs.
An early conventional notch sensor circuit 28 is illustrated in FIG. 2. As now known in the art, the light reflectance of the liner 22 will vary considerably between assorted types of label stock due to differences in material composition, color and manufacturing standards. More particularly, the conventional notch sensor circuit 28 includes a photodiode PD1 and phototransistor PQ1 that may be included in a common package, as described above. An anode terminal of the photodiode PD1 and a collector terminal of the phototransistor PQ1 are each coupled to a voltage source PV+ which provides a direct current voltage, through resistors PR1 and PR4, respectively. A cathode terminal of the photodiode PD1, and an emitter terminal of the phototransistor PQ1 are each coupled to ground. The collector terminal of the phototransistor PQ1 is further coupled to an operational amplifier which functions as a voltage comparator. A reference voltage is established by resistors PR2 and PR3 for the operational amplifier PU1C.
In operation of the notch sensor circuit 28, the voltage potential PV+ applied to the photodiode PD1, causing the photodiode to emit a steady, constant quantity of light determined by the fixed current level. The emitted light is reflected off the label stock as described above, a first portion of the light may be absorbed by the label stock and a second portion of the light impinges upon the phototransistor PQ1. The photo-transistor PQ1 becomes conductive in an amount proportional to the magnitude of light impinging thereon.
The light received by phototransistor PQ1 is converted to a voltage by resistor PR4 and directed to operational amplifier PU1C which functions as a voltage comparator. The voltage representing the light received is compared to a reference voltage which is established when the voltage potential PV+0 is applied to the operational amplifier PU1C through a resistor PR2 which is grounded through PR3. The output of the operational amplifier PU1C is either a high or low voltage representing a digital 1 or 0, respectively.
The accuracy of this output, however, will vary considerably depending on the optical environment, sensor characteristics or label stock composition. For example, if a printer with such a sensor was operated in an area of relatively large amounts of light, a notch edge may not be detected. Since the phototransistor receives a larger amount of light than expected, in relation to the reference voltage, the output corresponds to a value which would indicate that the motor should advance one more step, in other words, that there is no notch edge when one may actually be preset. Likewise, erroneous results occur if the label stock absorbs a considerable amount of the light, or the light output is diminished due to decay of the sensor or poor manufacturing standards.
Another device is cited as prior art in U.S. Pat. No. 5,693,931 to Wade in FIG. 2, which incorporates a potentiometer to permit the amount of current conducted through the phototransistor to be adjusted in order to calibrate the photosensor for differences in ambient light, color, and transmissivity of different materials. However, the adjustment of the potentiometer is a somewhat cumbersome task that is prone to error. Furthermore, the light emitted by the diode is of a steady, constant intensity.
Wade '931 claims a self-calibrating sensor circuit as described in FIG. 4 which does not require manual recalibration to increase or decrease the light output of the photosensor. Rather, the circuit automatically and dynamically calibrates itself for the changing operating conditions and for all types of print media.
A closed-loop control scheme is incorporated whereby an analog-to-digital converter communicates a value to the controller which represents the intensity of the light received by the photosensor. This loop includes a current regulator that controls the fixed amount of current drawn by the light emitting element. A summing junction combines the current from the light emitting and receiving elements and provides a combined current value to the current regulator. In other words, the feedback loop is used to monitor the output of the photoemitter, the reception of the photoreceiver and thereby sum their signals to reset the fixed current input. Wade '931 achieves the goals of automatically adjusting for variations in sensor characteristics and the optical environment much like the present invention, but in a much more complicated and expensive manner. Wade '931 uses more components, which also cost more to manufacture, in addition to the quantity, has more complex manufacturing and assembly and requires a more intricate routing required from the controller.
Another prior art reference worth noting is U.S. Pat. No. 5,248,879 to Turvy, Jr., which discloses another closed-loop control scheme. The output is monitored and used to adjust the control input. This interface uses a digital counter, a low-pass filter, and an amplifier to produce a fixed current through the photodiode, which results in a steady, constant light intensity. Additionally, an analog-to-digital converter is used to communicate a value to the controller which represents the intensity of the light received by the phototransistor. Turvy, Jr. '879 therefore achieves the same goals as Wade '931 in a different manner. However, in a manner more similar to Wade '931, Turvy, Jr. '879 uses more components, which in addition to the quantity, cost more to manufacture, has more complex manufacturing and assembly, and requires a more intricate routing required from the controller.
Thus, is it desirable to provide an apparatus for detecting a notch in a label stock that does not require manual or automatic recalibration to change the light intensity output of the photosensor to compensate for variations in sensor characteristics or the optical environment, and improvement in the art is needed. Such an apparatus should independently compensate for the changing ambient and sensor characteristic operating conditions and for all types of label stock, simply and without expensive and complicated feedback schemes.